Solution processable material for electronic and electro-optic applications

ABSTRACT

An electro-optic device has a first electrode, a second electrode spaced apart from the first electrode, an active layer disposed between the first electrode and the second electrode, and an interfacial layer in contact with the active layer. The interfacial layer is a blend of a metal oxide and a second material that at least one of reduces a work function or increases an electrical conductivity of the interfacial layer according to an embodiment of this invention. A composition for electro-optic devices is a blend of at least one metal oxide and at least one salt in a ratio, by volume, of at least 1:0.1 and less than 1:1.2.

BACKGROUND

1. Field of Invention

Embodiments of this invention relate to materials for electro-opticdevices, electro-optic devices that use the materials and methods ofproducing the materials and electro-optic devices. More particularly,embodiments of this invention relate to materials for electro-opticdevices that are blends of a metal oxide and at least one other materialthat increases electrical conductivity and/or decreases a work functionof a layer of such a material when blended with the metal oxide, toelectro-optic devices that use the materials and methods of producingthe materials and electro-optic devices.

2. Discussion of Related Art

The contents of all references referred to herein, including articles,published patent applications and patents are hereby incorporated byreference.

Electronic devices based on organic materials (small molecules andpolymers) have attracted broad interest. Such devices include organiclight emitting devices (OLEDs) (Tang, C. W.; VanSlyke, S. A. Appl. Phys.Lett. 1987, 51, 913), organic photovoltaic cells (OPVs) (Tang, C. W.Appl. Phys. Lett. 1986, 48, 183), transistors (Bao, Z.; Lovinger, A. J.;Dodabalapur, A. Appl. Phys. Lett. 1996, 69, 3066), bistable devices andmemory devices (Ma, L. P.; Liu, J.; Yang, Y. Appl. Phys. Lett. 2002, 80,2997), etc. Some of the most salient attributes of polymer electronicsis that they can be very low-cost, flexible, operate with low-energyconsumption, can be produced with high-throughput processing, and can beversatile for applications (Forrest, S. R. Nature 2004, 428, 911). Tofulfill the requirement of low cost, a solution process is highlydesirable. Within the field of organic electronics, the metal/organicinterface plays a critical role in determining the device performance.The interface can often be modified by some functional interfacial layerin order to improve device performance. Depending on the characteristicsof the material, the functional interfacial layer can be employed indifferent configurations. Early prominent examples in the development ofOLEDs and OPVs include—(1) Introducing LiF, CsF, AlO_(x), etc. as anelectron buffer layer in OLEDs (Hung, L. S.; Tang, C. W.; Mason, M. G.Appl. Phys. Lett. 1997, 70, 152; Grozea, D.; Turak, A.; Feng, X. D.; Lu,Z. H.; Johnson, D.; Wood, R. Appl. Phys. Lett. 2002, 81, 3173; Kin, Z.;Hino, Y.; Kajii, H.; Ohmori, Y. Mol. Cryst. Liq. Cryst., 2007, 462,225); (2) Application of polyaniline (PANI) (Yang, Y.; Heeger, A. J.Appl. Phys. Lett. 1994, 64, 1245) andpoly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) asa hole transport/buffer layer (Jonas, F.; Krafft, W.; Muys, B. Macromol.Symp. 1995, 100, 169); (3) Insertion of a TiO_(x) layer as an opticalspacer/hole blocking layer (Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.;Ma, W.; Gong, X.; Heeger, A. J. Adv. Mater. 2006, 18, 572; Hayakawa, A.;Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshikawa, S. Appl. Phys. Lett.2007, 90, 163517); (4) Combination of an n- and p-type transport layerfor tandem OLEDs (e.g. LiF-V₂O₅) (Chu, C.-W.; Chen, C.-W.; Li, S.-H.;Wu, E. H.-E.; Yang, Y. Appl. Phys. Lett. 2005, 86, 253503).

Solar cells, also known as photovoltaic (PV) cells or devices, generateelectrical power from incident light. The term “light” is used broadlyherein to refer to electromagnetic radiation which may include visible,ultraviolet and infrared light. Traditionally, PV cells have beenconstructed of a number of inorganic semiconductors, e.g., crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. More recently, PV cells have been constructedusing organic materials.

Solar cells are characterized by the efficiency with which they canconvert incident solar power to useful electric power. Devices utilizingcrystalline or amorphous silicon dominate commercial applications, andsome have achieved efficiencies of 23% or greater. However, efficientcrystalline-based devices, especially of large surface area, aredifficult and expensive to produce due to the problems inherent inproducing large crystals without significant efficiency-degradingdefects. On the other hand, high efficiency amorphous silicon devicesstill suffer from problems with stability. Present commerciallyavailable amorphous silicon cells have stabilized efficiencies between 4and 8%. More recent efforts have focused on the use of organicphotovoltaic cells to achieve acceptable photovoltaic conversionefficiencies with economical production costs as well as other possibleadvantageous properties.

PV devices produce a photo-generated voltage when they are connectedacross a load and are irradiated by light. When irradiated without anyexternal electronic load, a PV device generates its maximum possiblevoltage, V open-circuit, or V_(OC). If a PV device is irradiated withits electrical contacts shorted, a maximum short-circuit current, orI_(SC), is produced. (Current is conventionally referred to as “I” or“J”.) When actually used to generate power, a PV device is connected toa finite resistive load in which the power output is given by theproduct of the current and voltage, I×V. The maximum total powergenerated by a PV device is inherently incapable of exceeding theproduct I_(SC)×V_(OC). When the load value is optimized for maximumpower extraction, the current and voltage have values, I_(max) andV_(max), respectively. A figure of merit for solar cells is the fillfactor, ff (or FF), defined as:

${ff} = \frac{I_{\max}V_{\max}}{I_{SC}V_{OC}}$

where ff is always less than 1, as I_(SC) and V_(OC) are never achievedsimultaneously in actual use. Nonetheless, as ff approaches 1, thedevice is more efficient.

When electromagnetic radiation of an appropriate energy is incident upona semiconductive organic material, for example, an organic molecularcrystal (OMC) material, or a polymer, a photon can be absorbed toproduce an excited molecular state. This energy absorption is associatedwith the promotion of an electron from a bound state in the highestoccupied molecular orbital (HOMO) to the lowest unoccupied molecularorbital (LUMO), or equivalently, the promotion of a hole from the LUMOto the HOMO. In organic thin-film photoconductors, the generated excitedstate is believed to be an exciton, i.e., an electron-hole pair in abound state which is transported as a quasi-particle. The excitons canhave an appreciable life-time before recombination. To produce aphotocurrent the electron-hole pair must become separated, for exampleat a donor-acceptor interface between two dissimilar contacting organicthin films. The interface of these two materials is called aphotovoltaic heterojunction. If the charges do not separate, they canrecombine with each other (known as quenching) either radiatively, bythe emission of light of a lower energy than the incident light, ornon-radiatively, by the production of heat. Either of these outcomes isundesirable in a PV device. In traditional semiconductor theory,materials for forming PV heterojunctions have been denoted as generallybeing of either n (donor) type or p (acceptor) type. Here n-type denotesthat the majority carrier type is the electron. This could be viewed asthe material having many electrons in relatively free energy states. Thep-type denotes that the majority carrier type is the hole. Such materialhas many holes in relatively free energy states. The type of thebackground majority carrier concentration depends primarily onunintentional doping by defects or impurities. The type andconcentration of impurities determine the value of the Fermi energy, orlevel, within the gap between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO), called theHOMO-LUMO gap. The Fermi energy characterizes the statistical occupationof molecular quantum energy states denoted by the value of energy forwhich the probability of occupation is equal to ½. A Fermi energy nearthe LUMO energy indicates that electrons are the predominant carrier. AFermi energy near the HOMO energy indicates that holes are thepredominant carrier. Accordingly, the Fermi energy is a primarycharacterizing property of traditional semiconductors and the PVheterojunction has traditionally been the p-n interface.

A significant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. As opposed tofree carrier concentrations, carrier mobility is determined in largepart by intrinsic properties of the organic material such as crystalsymmetry and periodicity. Appropriate symmetry and periodicity canproduce higher quantum wavefunction overlap of HOMO levels producinghigher hole mobility, or similarly, higher overlap of LUMO levels toproduce higher electron mobility. Moreover, the donor or acceptor natureof an organic semiconductor may be at odds with the higher carriermobility. The result is that device configuration predictions fromdonor/acceptor criteria may not be borne out by actual deviceperformance. Due to these electronic properties of organic materials,the nomenclature of “hole-transporting-layer” (HTL) or“electron-transporting-layer” (ETL) is often used rather thandesignating them as “p-type” or “acceptor-type” and “n-type” or“donor-type”. In this designation scheme, an ETL will be preferentiallyelectron conducting and an HTL will be preferentially hole transporting.

Organic PV cells have many potential advantages when compared totraditional silicon-based devices. Organic PV cells are light weight,economical in the materials used, and can be deposited on low costsubstrates, such as flexible plastic foils. However, organic PV devicestypically have relatively low quantum yield (the ratio of photonsabsorbed to carrier pairs generated, or electromagnetic radiation toelectricity conversion efficiency), being on the order of 1% or less.This is, in part, thought to be due to the second order nature of theintrinsic photoconductive process. That is, carrier generation requiresexciton generation, diffusion and ionization. However, the diffusionlength (L_(D)) of an exciton is typically much less than the opticalabsorption length, requiring a trade off between using a thick, andtherefore resistive, cell with multiple or highly folded interfaces, ora thin cell with a low optical absorption efficiency.

For polymer solar cells, the polymer/fullerene based bulk-heterojunction(BHJ) solar cell is the most common device architecture (Li, G.;Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.Nat. Mater. 2005, 4, 864; Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis,S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.;McCulloch, I.; Ha, C.; Ree, M. Nat. Mater. 2006, 5, 197; Sievers, W. D.;Shrotriya, V.; Yang, Y. J. Appl. Phys. 2006, 100, 114509) for which arecently certified solar cell efficiency of 5.4% in a single cellconfiguration was achieved. Recently we showed that when salts likeCs₂CO₃ or CsF are applied as an n-type interfacial layer between thepolymer active layer and the aluminum electrode, photovoltage(open-circuit voltage Voc), fill-factor (FF) and device efficiency allimprove. In addition, application of Cs₂CO₃ (both by thermal evaporationand solution processes) as an effective electron injection/buffer layerleads to record high white and red PLEDs with significantly reduceddriving voltage and enhanced lifetime (Huang, J.; Xu, Z.; Yang, Y. Adv.Funct. Mater. 2007, 17, 1966). Combined with novel p-type interfaciallayers such as transition metal oxides (V₂O₅, MoO₃, WO₃ etc.), wesuccessfully demonstrated efficient inverted polymer solar cells (Li,G.; Chu, C.-W.; Shrotriya, V.; Huang, J.; Yang, Y. Appl. Phys. Lett.2006, 88, 253503; Shrotriya, V.; Li, G.; Yao, Y.; Chu, C.-W.; Yang, Y.Appl. Phys. Lett. 2006, 88, 073508). Furthermore, the sol-gel processtitanium sub-oxide (TiO_(x)) and zinc oxide (ZnO) were recently shown tobe an effective n-type buffer material for improving solar cellefficiency (Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.;Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864). However, there remains aneed for improved organic photovoltaic devices.

SUMMARY

An electro-optic device according to an embodiment of the currentinvention has a first electrode, a second electrode spaced apart fromthe first electrode, an active layer disposed between the firstelectrode and the second electrode, and an interfacial layer in contactwith the active layer. The interfacial layer comprises a blend of ametal oxide and a salt according to an embodiment of the currentinvention. The interfacial layer comprises a blend of a metal oxide anda second material that at least one of reduces a work function orincreases an electrical conductivity of the interfacial layer accordingto an embodiment of this invention.

A method of producing an electro-optic device according to an embodimentof the current invention includes forming an active polymer layer, andforming an interfacial layer on a surface of said active polymer layer.Forming the interfacial layer includes forming the interfacial layerfrom a blend of a metal oxide and a salt according to an embodiment ofthe current invention. Forming the interfacial layer includes formingthe interfacial layer from a blend of a metal oxide and a secondmaterial that at least one of reduces a work function or increases anelectrical conductivity of the interfacial layer according to anembodiment of this invention.

A composition according to an embodiment of the current invention is ablend of at least one metal oxide and at least one salt in a ratio, byvolume, of at least 1:0.1 and less than 1:1.2.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by reading the following detaileddescription with reference to the accompanying figures in which:

FIG. 1 is a schematic illustration of an electro-optic device accordingto an embodiment of the current invention;

FIG. 2 is a schematic illustration of an electro-optic device accordingto another embodiment of the current invention;

FIG. 3 is a schematic illustration of a photovoltaic cell according toan embodiment of the current invention;

FIG. 4 is a schematic illustration of an inverse photovoltaic cellaccording to an embodiment of the current invention;

FIG. 5 is a schematic illustration of a light-emitting diode accordingto an embodiment of the current invention;

FIG. 6 is a schematic illustration of a tandem photovoltaic cellaccording to an embodiment of the current invention;

FIGS. 7A and 7B show TEM images of TiO₂ and a mixed solution ofTiO₂:Cs₂CO₃, respectively;

FIG. 8 shows X-ray powder diffraction patterns for TiO₂ (bottom), andTiO₂:Cs₂CO₃ over view (middle) and zoomed in (top);

FIG. 9 shows XPS profiles of TiO₂ (dot line) and TiO₂:Cs₂CO₃ (solidline) samples for the Ti peak;

FIG. 10 shows a corresponding energy level diagram of a device based ona TiO₂:Cs₂CO₃ interfacial layer;

FIG. 11A shows J-V characteristics of a P3HT:PC₇₀BM based photovoltaiccell with an evaporated Al cathode and with different interfacial layers(none; Cs₂CO₃; TiO₂; TiO₂:Cs₂CO₃) and External Quantum efficiencies(EQE) of the device with and without the TiO₂:Cs₂CO₃ interfacial layer(inset);

FIG. 11B shows the J-V curve of an inverted solar cell with TiO₂:Cs₂CO₃in the dark and under an illumination of AM1.5;

FIG. 12 is a table summarizing photovoltaic performances ofcorresponding devices with different interfacial layers;

FIGS. 13A and 13B show CELIV extraction peaks and conductance data,respectively, for the /ITO/PEDOT/P3HT:PC₇₀BM/interfacial layer/Al devicewith Cs₂C O₃; TiO₂; TiO₂:Cs₂CO₃;

FIGS. 14A and 14B show Current density—Voltage—Brightnesscharacteristics and current efficiency, respectively, for theITO/PEDOT/LEP/EIL/Al device with different interfacial layers (none;Cs₂CO₃; TiO₂; TiO₂:Cs₂CO₃);

FIG. 15 shows Current Voltage characteristics of a photovoltaic deviceusing a TiO₂:CsF interfacial layer;

FIG. 16 shows Current-Voltage characteristics of the PF-co-DTB andMEHPPV based photovoltaic device with a TiO₂:Cs₂CO₃ layer;

FIGS. 17A and 17B show J-V characteristics of P3HT:PC₆₀BM basedphotovoltaic cells with an evaporated Al cathode and device photovoltaicparameters, respectively, with various TiO₂:Cs₂CO₃ mixing ratios; and

FIG. 18 shows Current density-Voltage characteristics under AM 1.5Gconditions and in the dark, indicating that the TiO₂:Cs₂CO₃:PEDOT layerserves as a good recombination site for both charge carriers.

DETAILED DESCRIPTION

In describing embodiments of the present invention, specific terminologyis employed for the sake of clarity. However, the invention is notintended to be limited to the specific terminology so selected. It is tobe understood that each specific element includes all technicalequivalents which operate in a similar manner to accomplish a similarpurpose.

FIG. 1 is a schematic illustration of an electro-optic device 100according to an embodiment of the current invention. The electro-opticdevice 100 has a first electrode 102, a second electrode 104 spacedapart from the first electrode 102, and an active layer 106 disposedbetween the first electrode and the second electrode. The electro-opticdevice 100 also has an interfacial layer 108 in contact with the activelayer 106. The term active layer is intended to have a broad meaningthat includes a layer of material that can absorb light to provide acurrent and/or can emit light in response to an electrical current. Forexample, the active layer may include organic and/or inorganicsemiconductors. In addition, although the interfacial layer 108 isillustrated as being on the side of the active layer 106 that is closestto the second electrode 104 in the example of FIG. 1, it could also belocated on the other side of the active layer 106 that is closest to thefirst electrode 102 in other embodiments of the current invention.Furthermore, the invention is not limited to only active layer 106 andinterfacial layer 108 between the first electrode 102 and secondelectrode 104. Additional active layers, additional interfacial layersand/or other layers of material may also be included between the firstelectrode 102 and the second electrode 104 according to some embodimentsof the current invention. Some specific examples will be described belowin this specification, without limitation, that have additional layersbetween first and second electrodes.

The interfacial layer 106 is a blend of a metal oxide and at least oneother material that provides at least one of a decrease in the workfunction or an increase of electrical conductivity of the interfaciallayer 106 compared to the metal oxide alone. For example, theinterfacial layer 106 can be a blend of a metal oxide and a saltaccording to an embodiment of the current invention. In one example, themetal oxide can be titanium oxide and the salt can be cesium carbonate.In an embodiment the interfacial layer 106 can be a blend of titaniumoxide and cesium carbonate in a ratio of at least about 1:0.1 and lessthan about 1:1.2. In an embodiment the interfacial layer 106 can be ablend of titanium oxide and cesium carbonate in a ratio of about 1:1.However, the at least one additional material in the blend for theinterfacial layer 106 is not limited to only salts. It may also includea metal composite consisting of metal salt and/or metal ions, or adoping component. The metal element of the composite can include, but isnot limited to, Pt, Pd, Ni and Au. The metal composition can be anycompound having at least one element selected from the list above. Thedoping component can be at least one element selected from the groupconsisting of O, N, Fe, V, Nb and W. In addition, the salt is notlimited to only cesium carbonate. It may also include one or more alkalimetal carbonate and/or alkali metal fluoride consisting of Li, Na, K, Rband Cs, or combinations thereof.

FIG. 2 is a schematic illustration of an electro-optic device 200according to another embodiment of the current invention. Theelectro-optic device 200 has a first electrode 202, a second electrode204 spaced apart from the first electrode 202, and an active layer 206disposed between the first electrode and the second electrode. Theelectro-optic device 200 also has an interfacial layer 208 in contactwith the active layer 206. This embodiment is an example of anelectro-optic device that has a second active layer 210 between thefirst electrode 202 and the second electrode 204. In this case, there isan interfacial layer 208 that is sandwiched between and in contact withtwo active layers.

FIG. 3 is a schematic illustration of an electro-optic device 300according to another embodiment of the current invention. Theelectro-optic device 300 has a first electrode 302, a second electrode304 spaced apart from the first electrode 302, and an active layer 306disposed between the first electrode and the second electrode. Theelectro-optic device 300 also has an interfacial layer 308 in contactwith the active layer 306 and the second electrode 304. In thisembodiment, the first electrode 302 is a transparent electrode and theelectro-optic device is a photovoltaic cell. The electro-optic device300 also has a hole transport layer 310 arranged between and in contactwith the active layer 306 and the first electrode 302.

FIG. 4 is a schematic illustration of an electro-optic device 400according to another embodiment of the current invention. Theelectro-optic device 400 has a first electrode 402, a second electrode404 spaced apart from the first electrode 402, and an active layer 406disposed between the first electrode and the second electrode. Theelectro-optic device 400 also has an interfacial layer 408 in contactwith the active layer 406 and the first electrode 402. In thisembodiment, the first electrode 402 is a transparent electrode and theelectro-optic device is an inverted photovoltaic cell. The electro-opticdevice 400 also has a hole buffer layer 410 arranged between and incontact with the active layer 406 and the second electrode 404.

FIG. 5 is a schematic illustration of an electro-optic device 500according to another embodiment of the current invention. Theelectro-optic device 500 has a first electrode 502, a second electrode504 spaced apart from the first electrode 502, and an active layer 506disposed between the first electrode and the second electrode. Theelectro-optic device 500 also has an interfacial layer 508 in contactwith the active layer 506 and the second electrode 504. In thisembodiment, the electro-optic device is a polymer light-emitting diode.The electro-optic device 500 also has a hole transport layer 510arranged between and in contact with the active layer 506 and the firstelectrode 502.

FIG. 6 is a schematic illustration of an electro-optic device 600according to another embodiment of the current invention. Theelectro-optic device 600 has a first electrode 602, a second electrode604 spaced apart from the first electrode 602, and an active layer 606disposed between the first electrode and the second electrode. Theelectro-optic device 600 also has an interfacial layer 608 in contactwith the active layer 606. The electro-optic device 600 has a secondactive layer 610 that is in contact with the interfacial layer such thatthe interfacial layer is sandwiched between the active layers 606 and610. In this embodiment, the electro-optic device is a tandemphotovoltaic cell. The electro-optic device 600 also has a holetransport layer 612 arranged between and in contact with the activelayer 606 and the first electrode 602.

EXAMPLES

According to an embodiment of the current invention, we provide a methodto make efficient organic electronic devices by utilizing a mixture ofsolution processable semiconducting metal oxides and salts to formfunctional interfacial layers in polymer electronics. One example is ananatase TiO₂ and Cs₂CO₃ blend. Introducing a nano-scale TiO₂ and Cs₂CO₃interfacial layer can provide solar cell performance as good as a devicewith a low work-function metal Ca as an electrode which is easy to beoxidized (Greczynski, G.; Kugler, T.; Keil, M.; Osikowicz, W.; Fahlman,M.; Salaneck, W. R. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 1).The effectiveness of this unique approach is expanded to polymer LEDs inother particular embodiments of this invention where a lower drivingvoltage, improved efficiency and extended lifetime are againdemonstrated.

Semiconducting TiO₂ has been extensively studied as a promising materialin a variety of applications including dye-sensitized solar cells,sensitization, organic photovoltaics (O'Regan, B.; Grätzel, M. Nature1991, 353, 737; Oey, C. C.; Djuri{hacek over (s)}i{hacek over (c)}, A.B.; Wang, H.; Man, K. K. Y.; Chan, W. K.; Xie, M. H.; Leung, Y. H.;Pandey, A.; Nunzi, J.-M.; Chui, P. C. Nanotechnology 2006, 17, 706). Itis widely accepted that the application of nanocrystalline TiO₂ tophotovoltaics remains limited due to the hydrothermal processing orcalcinations to induce crystallization (Niederberger, M.; Bartl, M. H.;Stucky, G. D. Chem. Mater. 2002, 14, 4364). A feature of TiO₂ accordingto some embodiments of the current invention is the use of thenanocrystalline anatase form produced by a nonhydrolitic sol-gel processat low temperatures. This approach can provide several importantadvantages in some embodiments: (a) the elimination of additional agentsallows low particle agglomeration; (b) no exposure to air is needed; and(c) the elimination of water enables the formation of homogeneous films(Niederberger, M.; Bartl, M. H.; Stucky, G. D. Chem. Mater. 2002, 14,4364). It is worth noting that the anatase crystalline form was obtainedeven with the as-prepared sol-gel TiO₂, which is soluble in certainsolvents at room temperature. This eliminates the high temperaturesintering process (400-500° C.) which otherwise inhibits the applicationof crystalline TiO₂'s in regular OPV structures. The synthesis ofcrystalline TiO₂ nano-particles follows a method that can be found inthe literature (Wang, J.; Polleux, J.; Lim, J.; Dunn, B. J. Phys. Chem.C 2007, 111,14925). All chemicals were purchased from Sigma-Aldrich andused as received. 0.5 mL of TiCl₄ was slowly added into 2 mL of ethanoland mixed with 10 mL of benzyl alcohol. After stirring the solution for9 hrs at 80° C., 12 mL of diethyl ether for every 1 mL of solution wasused for washing and the white color TiO₂ precipitate was obtained fromcentrifuging the crude product. The final solution was prepared bydispersing in ethanol. A solution of TiO₂ and Cs₂CO₃ was obtained byblending 0.2 wt % of Cs₂CO₃ in 2-ethoxyethanol solution with TiO₂solution (0.2 wt %) in a 1:1 volume ratio. The solution was used aftervigorous stirring for several hours. Transmission electron microscopy(TEM, JEOL JEM-1200EX) images of TiO₂ and the mixture of TiO₂:Cs₂CO₃ areshown in FIGS. 7A and 7B, respectively. An overview image of TiO₂nanoparticles illustrates that the material is entirely composed ofnano-sized particles homogeneously distributed throughout the material.As we continue to blend with Cs₂CO₃, TEM images show that the productconsists of markedly more monodispersed shapes. The TEM image (notshown) of the solution mixture taken one week after being exposed to airshows that the mixture is stable and the product has not agglomerateddue to the addition of Cs₂CO₃, which has a stabilizing effect on thesolution by preventing the three dimensional titania network fromshrinkage, which is observed in pure TiO₂ solutions (Wang, J.; Polleux,J.; Lim, J.; Dunn, B. J. Phys. Chem. C 2007, 111,14925).

The crystalline phase evolution of these two samples was monitored withan X-ray powder diffractometer (PANalytical X'Pert Pro). The powdersamples with the composition of TiO₂ and TiO₂:Cs₂CO₃ for X-raydiffraction (XRD) analysis were prepared by evaporating the solvent at110° C. in an oven. FIG. 8 shows the XRD data. The X-ray powderdiffraction pattern for TiO₂ obtained by the sol-gel method confirms theexistence of nano-crystalline TiO₂ in the anatase form, which agreeswell with literature (Niederberger, M.; Bartl, M. H.; Stucky, G. D.Chem. Mater. 2002, 14, 4364; Wang, J.; Polleux, J.; Lim, J.; Dunn, B. J.Phys. Chem. C 2007, 111,14925). All the peaks are ascribed to theanatase crystal structure without any secondary reaction impurities. Theindexed broad peaks indicate the nano-crystalline nature of TiO₂ withsize between 7-8 nm. The XRD spectrum of TiO₂:Cs₂CO₃ and enlargedspectrum are shown in FIG. 8. When Cs₂CO₃ is added to sol-gel TiO₂, theXRD spectra are assigned by the peak pattern for the anatase phase ofTiO₂ as well as CsCl cubic structure, which can be formed by reaction ofresidual benzyl chloride as side products from the stock TiO₂ solutionwith Cs₂CO₃. The narrowed peak width of CsCl shows the characteristicsof highly ordered crystalline CsCl compared to TiO₂. On the other hand,the existence of nano-crystalline anatase TiO₂ in the TiO₂:Cs₂CO₃ sampleis evidenced by the similar peak width and intensity from the enlargedXRD data.

X-ray photoemission spectroscopy (XPS) was performed in an OmicronNanotechnology system to further investigate the surface characteristicsof TiO₂ and the Cs₂CO₃ interfacial layer. The data are shown in FIG. 9.The samples are prepared from spin casting on an Ag coated Si wafer andthe instrument was calibrated using an internal Ag standard. The atomicratio of oxygen to titanium was estimated to be 1.99 based on theintegrated area under the element peak and sensitivity factor for theelement, calibrated using commercially available crystalline TiO₂ powderas a reference (Sigma Aldrich, used as received). The data implies thattitanium dioxide prepared from the non-hydrolytic sol-gel method ischemically stoichiometric, which is also in good agreement with thepreviously reported Ti 2p_(3/2) peak position for TiO₂ (Atashbar, M. Z.;Sun, H. T.; Gong, B.; Wlodarski, W.; Lamb, R. Thin Solid Films 1998,326, 238). We note that the Ti 2p_(3/2) spectra for TiO₂:Cs₂CO₃ solutionshifts towards lower binding energy by 0.78 eV as compared to the valuefor TiO₂. We suspect that this change is due to the presence of Ti-O-Csbonds, which is supported by studies on a Fe-doped SiO₂ system (Ingo, G.M.; Dire, S.; Babonneau, F. Appl. Surf. Science 1993, 70, 230). It isalso consistent with previous reports that the Ti(2p) peaks shiftconsiderably to lower binding energy upon Cs or K adsorption (Grant, A.W.; Campbell, C. T. Phys. Review. B 1997, 55, 1844; Hardman, P. J.;Casanova, R.; Prabhakaran, K.; Muryn, C. A.; Wincott, P. J.; Thornton,G. Surf Sci. 1992, 269, 677). The Cs 3d_(5/2) peak position ofTiO₂:Cs₂CO₃ shifted toward low binding energy, compared to that ofCs₂CO₃ also supports the bonding formation between TiO₂ and Cs. Metalsin an organic/inorganic matrix can act as a doping component. Thisdiscrepancy of XPS survey spectra may be explained by the possibility offormation of Cs-doped TiO₂ materials. The energy level of both TiO₂ andTiO₂:Cs₂CO₃ samples have been determined with electrochemical cyclicvoltammetry (C-V) and the energy offset wavelength on UV-vis absorptionspectra. The energy level diagram is shown in the FIG. 10

The photovoltaic devices in the current examples were fabricated basedon a blend of poly (3-hexylthiophene) (P3HT) and[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) system using Al as acathode as well as insertion of Cs₂CO₃ and TiO₂ and mixture ofTiO₂:Cs₂CO₃ as an interfacial layer between an active layer and thecathode. The polymer blend of P3HT: PC₇₀BM was spin casted on thepoly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS)deposited ITO glass, followed by a thermal annealing at 110° C. Theinterfacial layer was spin cast from each solution and the film was thenannealed at 80° C. The device fabrication was completed by thermalevaporation of 100 nm of Al as a cathode. The current density-voltage(J-V) characteristics under AM 1.5G one-sun illumination is shown inFIG. 11A and the summary of the device performance is listed in thetable in FIG. 12. Comparison of devices with an Al electrode to thatwith Cs₂CO₃/Al shows decreasing Voc and Jsc upon the insertion of aspin-casted film of Cs₂CO₃. This implies that a Cs₂CO₃-only interfaciallayer doesn't provide the appropriate function in terms of chargeextraction and transport to the electrode. The insertion of a TiO₂ layerbetween the active layer and the evaporated Al cathode layer leads to anincrease in Voc up to 0.46 eV. This may be due to the work function ofTiO₂. It is known that open circuit voltage is generally determined bythe difference between the highest occupied molecular orbital (HOMO) ofdonor and the lowest unoccupied molecular orbital (LUMO) of acceptor inthe case of Ohmic contact between an active layer and a cathode (Brabec,C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz, T.;Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv. Funct. Mater. 2001,11, 374). The conduction band level of TiO₂ is 4.3 eV as determined froma C-V experiment, which is slightly higher than the work function of theAl electrode. This brings unfavorable electron charge extraction fromthe active layer to electrode and an S-shape of the J-V curve isobserved. On the other hand, we clearly see an improvement in Voc, Jscand FF for the devices with a functional TiO₂:Cs₂CO₃ mixed film,consequently, resulting in efficient device performance. Voc increasedfrom 0.42 V (for Al only device) to 0.58 V and FF is also dramaticallyimproved up to 67%, giving power conversion efficiency of 4.2%, which isquite comparable to a device with a Ca/Al electrode. One possible reasonmay be attributed to the formation of the better Ohmic contact createdby the decreased conduction band of the TiO₂:Cs₂CO₃ mixture layer (3.93eV) such that the interfacial layer facilitates electron transport fromthe active layer to the cathode. Under dark conditions, therectification ratios are ˜10⁶ and serial resistance is considerablydecreased to 1-2 Ω·cm², while shunt resistance remains as high as closeto 10⁷ Ω·cm², which makes it ideal for photovoltaics. It is believedthat the TiO₂:Cs₂CO₃ layer can keep the hot Al electrode away fromdiffusion to the active layer during evaporation and offer good contactmorphology between active layer and electrode. This is also supported bythe dark current characteristics of the device with only Al where theyhave similar shunt resistance but higher serial resistance by severaltens Ω·cm². In addition, the highly negative valence band location ofthe interfacial layer serves as an efficient hole blocking layer, whichis confirmed by the small leakage current for TiO₂:Cs₂CO₃ based device.The External Quantum Efficiency (EQE) of the device using TiO₂:Cs₂CO₃ asan interfacial layer is shown in FIG. 11A (inset) as compared to thereference device with an Al electrode, which is consistent with thecurrent density—voltage (J-V) characteristic. We note that the deviceutilizing only a CsCl interfacial layer doesn't correspond to anyimproved device characteristics including high Voc, FF and small serialresistance. This indicates that the CsCl layer does not play any directrole in efficiency improvement, although the CsCl component in XRD dataseems to be a major component. Instead, nanocrystalline anatase phasesderived from TiO₂ such as doped TiO₂ is likely to be a reason forefficiency enhancement.

An inverted structure was investigated in polymer solar cells usingCs₂CO₃ to modify the ITO electrode as a cathode and using a transitionmetal oxide V₂O₅ as a hole buffer layer. We reported inverted solarcells with an efficiency of 2.25% due to a non-optimized interfaciallayer and active layer processes (Li, G.; Chu, C.-W.; Shrotriya, V.;Huang, J.; Yang, Y. Appl. Phys. Lett. 2006, 88, 253503). Thicker bufferlayers on top of active materials can be applied in inverted cells, sothe structure is more robust to transparent electrode deposition, e.g.ITO sputtering. A lamination process fabrication of semitransparent andflexible solar cells was recently shown based on the same interfacemodification approach (Huang, J.; Li, G.; Yang, Y. Adv. Mater. 2008, 20,415). Here we applied a TiO₂:Cs₂CO₃ mixture to replace Cs₂CO₃ anddemonstrate the fabrication of highly efficient inverted polymer solarcells based on P3HT and PCBM. The sequence of the inverted devicestructure is as follows: /ITO/TiO₂:Cs₂CO₃/P3HT:PC₇₀BM/V₂O₅/Al. TheTiO₂:Cs₂CO₃ layer was spin cast on ITO coated glass and thermal heattreatment was performed at 150 degrees for 30 min. After spin casting apolymer blend solution of P3HT:PC₇₀BM, followed by another thermalannealing at 110 degrees for 10 min, the device fabrication wascompleted by thermal evaporation of 5 nm of V₂O₅ and 80 nm of Al as ananode. The dark and photo (AM1.5, 100 mA/cm²) J-V curves of the devicewith TiO₂:Cs₂CO₃ mixture interfacial layer are shown in FIG. 11B. Forthe device with a TiO₂:Cs₂CO₃ layer, optimization in the devicefabrication again leads to improvements in Voc and FF, whichsubsequently results in a device efficiency of 3.9%, with Voc, Jsc andFF being 0.60V, 11.5 mA/cm² and 57%, respectively. The highrectification ratio is also attributed to the improved injection currentat the forward bias as shown in the dark current.

We used the Charge Extraction by Linearly Increasing Voltage (CELIV)method to investigate the charge carrier transport characteristics ofthe TiO₂:Cs₂CO₃ layer for some of representative regular configurationdevices. In CELIV, the very initial rise speed is caused by the bulkconductivity of the sample and the time of extraction current maximumt_(max) is used for the estimation of the drift mobility of equilibriumcharge carriers (Juska, G.; Arlauskas, K.; Viliūnas, M.; Genevi{hacekover (c)}ius, K.; Osterbacka, R.; Stubb, H. Phys. Rev. B 2000, 62,R16235). Under a ramping speed of 10⁵ V/cm, CELIV extraction peaks wereobtained as shown in FIG. 13A. The change in t_(max) is negligible andcorrespondingly little difference was observed in the mobility valuesfor the different devices being of the order of 10⁻⁴ cm²/Vs for alldevices. Impedance spectroscopy was used to measure the bulkconductivity of the samples. All our devices fitted well to theR_(p)-C_(p) (resistor-capacitor in parallel) model wherein theconductance

$\left( {G = \frac{1}{R_{p}}} \right)$

should be independent of the frequency and the susceptance(B=j(2π*f*C_(p))) should vary linearly with frequency. Conductance dataderived in this way is shown in FIG. 13B. The conductance forTiO₂:Cs₂CO₃ film was at least three orders of magnitude higher than thecorresponding Cs₂CO₃/Al or TiO₂/Al devices. Since the number of chargesextracted is directly proportional to the ratio of conductivity dividedby mobility, we conclude that the devices with TiO₂:Cs₂CO₃ improvecharge extraction from the polymer active layer. Additional support forthis argument comes from the CELIV data. The area under the currentdensity-time curve is the sum of the capacitive charges and theequilibrium charges extracted from the device. Subtracting thecapacitive charges (initial current rise j(0)), we see that the area forthe TiO₂:Cs₂CO₃/Al device is larger than the corresponding TiO₂/Aldevice. The equilibrium charge carriers extracted under dark for theTiO₂:Cs₂CO₃ devices are hence larger than that for TiO₂/Al devices.

In an effort to explore the effectiveness of the TiO₂ and Cs₂CO₃ mixturelayer, green-polyfluorene based PLEDs were constructed with thestructure/ITO/PEDOT:PSS(40 nm)/light emitting polymer (LEP) (80nm)/interfacial layer/Al, where the interfacial layer is (a) TiO₂, (b)Cs₂CO₃ and (c) TiO₂:Cs₂CO₃ mixture, all in 2-ethoxyethanol as solvent.To exclude the solvent effect on device performance, the solvent itselfis spin-cast between LEP and Al to make a reference diode. FIG. 14Ashows the comparison of current density—voltage—brightness (J-V-L)characteristics for the effects of different interfacial layers ondevice performance. The Cs₂CO₃ interfacial layer has been shown to be aneffective electron-injection layer which leads to white and red emissionPLEDs with record high power efficiency (Huang, J.; Li, G.; Wu, E.; Xu,Q.; Yang, Y. Adv. Mater. 2006, 18, 114; Huang, J.; Watanabe, T.; Ueno,K.; Yang, Y. Adv. Mater. 2007, 19, 739). The significant improvements indevice performance have been attributed to the formation of a lowwork-function complex and surface dipole which can facilitate electroninjection from the cathode (Huang, J.; Xu, Z.; Yang, Y. Adv. Funct.Mater. 2007, 17, 1966). Surprisingly, with the interfacial layer ofTiO₂:Cs₂CO₃ mixture, further improvements in both current density andbrightness are observed as compared to the Cs₂CO₃ and TiO₂ interfaciallayer leading to a current efficiency of 11.5 cd/A or power efficiencyof 14 μm/w at a bias of 2.8 V, as shown in FIG. 14B. The turn-on voltage(around 2.3 V) does not change, and implies that our PLEDs with theinterfacial layer of TiO₂:Cs₂CO₃ mixture may not further lower theelectron injection barrier compared with reference devices. However, theincreases of current density and brightness suggest that better chargebalance should be responsible for the efficiency enhancement. Asdiscussed above, a nearly Ohmic contact is observed with the interfacelayer of TiO₂:Cs₂CO₃ mixture, supported by the conductivity and energylevel alignment between the organic materials and the metal cathode.Moreover, the Valence Band level (7.6 eV) of the mixture interface layeris lower than the HOMO (5.4 eV) level of the organic active layer,providing a hole-blocking effect in our device structure. By combiningthe effect of Ohmic contact and hole-blocking effects, better chargebalance and higher device performance can be achieved. Therefore,compared with PLEDs with only a Cs₂CO₃ or TiO₂ interfacial layer, themixture layer shows both characteristics of the lower work function andhole-blocking effect from Cs₂CO₃ and TiO₂, respectively.

A mixture of TiO₂:Cs₂CO₃ has been shown to have superior performance inboth polymer solar cells and PLEDs. The concept of metal oxide and saltmixture as a functional interfacial layer for organic electronicdevices, however, is quite broad. For example, Cs₂CO₃ can be expanded toother kinds of carbonate compounds and to other types of salts having alow-work function metal ion in it. As an example, FIG. 15 shows the J-Vcurves of ITO/PEDOT:PSS/P3HT:PCBM/interfacial layer/Al devices with theinterfacial layer being (a) none; (b) CsF; (c) TiO₂; and (d) TiO₂:CsFblend. Although the photocurrent is reduced, the much improved Voc andFF for the device with a TiO₂:CsF interlayer leads to significantimprovements in device performance with the PCE of 3.1%, rather thanusing a single component interfacial layer; 26% compares to TiO₂ and 63%to CsF. The exact reason why the device with TiO₂:Cs₂CO₃ shows highercurrent than one with CsF is still an open question. It could be relatedto the interaction between the two components in the mixture. Furtherinvestigation is required to clarify the mechanism. However, the currentinvention is not limited by the theoretical explanation of the specificmechanism. Combinations of salt and solution processable metal oxidesare applicable to other conjugated polymers in addition to P3HT: (e.g.poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5′,5′-diyl}(PF-co-DTB), poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV),poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT) etc.) according to other embodiments of the current invention.We, indeed, have demonstrated the effectiveness of TiO₂:Cs₂CO₃ layers onrepresentative polymer:PCBM solar cell systems (PF-co-DTB & MEHPPV) inFIG. 16. A clear increase in Voc, rectification ratio and especially FFare observed.

We have demonstrated a novel approach for fabricating efficient organicelectronic devices by utilizing a mixture of solution processable metaloxides and salts as an interfacial layer. The nano-crystalline TiO₂ issynthesized using a nonhydrolitic sol-gel approach and is mixed with aCs₂CO₃ solution. Polymer solar cells based on P3HT:PC₇₀BM with aTiO₂:Cs₂CO₃ mixture interfacial layer have reached a power conversionefficiency of 4.2% in regular configurations. Significant improvementsin PLED performance have also been provided.

FIG. 17A shows J-V characteristic of a P3HT:PC₆₀BM based photovoltaiccell with an evaporated Al cathode for various TiO₂:Cs₂CO₃ mixingratios. FIG. 17B shows device photovoltaic parameters with variousmixing TiO₂:Cs₂CO₃ ratios. According to an embodiment of the currentinvention, a composition consists essentially of a blend at least onemetal oxide and at least one salt in a ratio, by volume, of at least1:0.1 and less than 1:1.2. According to another embodiment of thecurrent invention, a composition consists essentially of a ratio ofmetal oxide to salt is about 1:1.

FIG. 18 shows data for an embodiment of a tandem structure that has beendemonstrated as an effective approach to enhance a polymer solar cellperformance, especially photovoltage. One of the key elements inconstructing efficient tandem devices is an interconnection unit whichhas to function as a good charge recombination site and at the same timeprotect the bottom cell from damage during the film formation process ofthe top cell. We have demonstrated an embodiment of a tandem cellutilizing a nano-crystalline TiO₂:Cs₂CO₃ n-type interfacial layer, withthe active materials being a low band-gap polymerpoly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT): PCBM and P3HT:PC₇₀BM for front and rear cells, respectively.The device structure is as follows:

/ITO/PEDOT/PCPDTBT:PCBM/TiO₂:Cs₂CO₃/PEDOT/P3HT:PC₇₀BM/Al.

Current density-Voltage characteristics under AM 1.5G conditions and inthe dark is shown in FIG. 18, indicating that the TiO₂-Cs₂CO₃:PEDOTlayer serves as a good recombination site for both charge carriers.

The current invention was described with reference to particularembodiments and examples. However, this invention is not limited to onlythe embodiments and examples described. One of ordinary skill in the artshould recognize, based on the teachings herein, that numerousmodifications and substitutions can be made without departing from thescope of the invention which is defined by the claims.

1. An electro-optic device, comprising: a first electrode; a secondelectrode spaced apart from said first electrode; an active layerdisposed between said first electrode and said second electrode; and aninterfacial layer in contact with said active layer, wherein saidinterfacial layer comprises a blend of a metal oxide and a salt.
 2. Anelectro-optic device according to claim 1, wherein said interfaciallayer consists essentially of said blend of said metal oxide and saidsalt.
 3. An electro-optic device according to claim 1, wherein saidmetal oxide is titanium oxide and said salt is cesium carbonate.
 4. Anelectro-optic device according to claim 2, wherein said metal oxide istitanium oxide and said salt is cesium carbonate.
 5. An electro-opticdevice according to claim 1, wherein said metal oxide consistsessentially of metal-oxide nanoparticles.
 6. An electro-optic deviceaccording to claim 5, wherein said metal-oxide nanoparticles aretitanium oxide nanoparticles and said salt is cesium carbonate.
 7. Anelectro-optic device according to claim 2, wherein said metal oxideconsists essentially of metal-oxide nanoparticles.
 8. An electro-opticdevice according to claim 7, wherein said metal-oxide nanoparticles aretitanium oxide nanoparticles and said salt is cesium carbonate.
 9. Anelectro-optic device according to claim 1, wherein said metal oxide isselected from the group of metal oxides consisting of titanium oxide,zinc oxide, nickel oxide, molybdenum oxide, hafnium oxide, vanadiumoxide and any combination thereof.
 10. An electro-optic device accordingto claim 1, wherein said salt is selected from the group of saltsconsisting of an alkali metal carbonate and an alkali metal fluorideconsisting of Li, Na, K, Rb and Cs, or combinations thereof.
 11. Anelectro-optic device according to claim 10, wherein said salt isselected from the group of salts consisting of cesium carbonate, cesiumfluoride, lithium carbonate, sodium carbonate, potassium carbonate andany combination thereof.
 12. An electro-optic device according to claim1, wherein said active layer is a polymer active layer.
 13. Anelectro-optic device according to claim 1, wherein said first electrodeis a transparent electrode, said active layer is an active polymerlayer, and said interfacial layer is arranged on a side of said activelayer closer to said second electrode than to said first electrode, saidelectro-optic device being a polymer photo-voltaic device.
 14. Anelectro-optic device according to claim 13, further comprising a holetransport layer arranged between and in contact with said firstelectrode and said active layer.
 15. An electro-optic device accordingto claim 14, wherein said active layer consists essentially of a blendof ploy(3-hexylthiophene) and fullerenes, said interfacial layerconsists essentially of titanium oxide and cesium carbonate, and saidhole transport layer consists essentially of PEDOT.
 16. An electro-opticdevice according to claim 1, wherein said first electrode is atransparent electrode, said active layer is an active polymer layer, andsaid interfacial layer is arranged on a side of said active layer closerto said first electrode than to said second electrode, saidelectro-optic device being a polymer inverted photo-voltaic device. 17.An electro-optic device according to claim 16, further comprising a holebuffer layer arranged between and in contact with said second electrodeand said active layer.
 18. An electro-optic device according to claim17, wherein said active layer consists essentially of a blend ofploy(3-hexylthiophene) and fullerenes, said interfacial layer consistsessentially of a blend of titanium oxide and cesium carbonate, and saidhole transport layer consists essentially of vanadium oxide.
 19. Anelectro-optic device according to claim 1, wherein said first electrodeis a transparent electrode, said active layer is a light-emittingpolymer layer, and said interfacial layer is arranged on a side of saidactive layer closer to said second electrode than to said firstelectrode, said electro-optic device being a polymer light-emittingdiode.
 20. An electro-optic device according to claim 19, furthercomprising a hole transport layer arranged between and in contact withsaid first electrode and said active layer.
 21. An electro-optic deviceaccording to claim 20, wherein said active layer consists essentially ofa light emitting polymer layer, said interfacial layer consistsessentially of a blend of titanium oxide and cesium carbonate, and saidhole transport layer consists essentially ofpoly(3,4-ethylenedioxythiophene):poly(styrene sulfonate).
 22. Anelectro-optic device according to claim 1, further comprising a secondactive layer arranged between said first and second electrodes and incontact with said interfacial layer such that said interfacial layer isan intermediate layer between and in contact with the first mentionedand the second active layers.
 23. An electro-optic device, comprising: afirst electrode; a second electrode spaced apart from said firstelectrode; an active layer disposed between said first electrode andsaid second electrode; and an interfacial layer in contact with saidactive layer, wherein said interfacial layer comprises a blend of ametal oxide and a second material that at least one of reduces a workfunction or increases an electrical conductivity of the interfaciallayer.
 24. A method of producing an electro-optic device, comprising:forming an active polymer layer; and forming an interfacial layer on asurface of said active polymer layer, wherein said forming saidinterfacial layer comprises forming said interfacial layer from a blendof a metal oxide and a salt.
 25. A method of producing an electro-opticdevice according to claim 24, further comprising producing saidcomposition of said metal oxide and said salt by a sol-gel process. 26.A method of producing an electro-optic device according to claim 25,wherein said sol-gel process comprises: forming a solution of a metaloxide in an anhydrous solvent; forming a solution of a salt in ananhydrous solvent; and blending said metal oxide and said saltsolutions.
 27. A composition consisting essentially of a blend at leastone metal oxide and at least one salt in a ratio, by volume, of at least1:0.1 and less than 1:1.2.
 28. A composition according to claim 27,where said ratio of said metal oxide to said salt is about 1:1.
 29. Acomposition according to claim 27, where said metal oxide is titaniumoxide and said salt is cesium carbonate.
 30. A composition according toclaim 28, where said metal oxide is titanium oxide and said salt iscesium carbonate.
 31. A composition according to claim 27, where saidmetal oxide comprises a plurality of nano-crystalline nanoparticles.